Three explosives that shattered windows and injured a player on the Borussia Dortmund team bus Tuesday as the German football squad was en route to its home Champions League match were part of a targeted attack, police said.Investigators found near the scene a handwritten letter that claimed responsibility for the attack, prosecutor Sandra Luecke said.One explosive device failed to go off, Luecke said.Dortmund police Chief Gregor Lange said police have not ruled out any possible motives for the attack, which led to the match being postponed until Wednesday night.

Twenty-six-year-old Spanish defender Marc Bartra suffered injuries to an arm and hand, the team said. Bartra, who has played 17 games in all competitions for Dortmund this season, was treated at the scene and hospitalized.

The team later said Bartra was in surgery for a broken radius -- a bone in the forearm -- and for "bits of debris lodged in his hand." He will not play Wednesday."We hope that he will make a speedy recovery," Dortmund captain Marcel Schmelzer said on the team website. "The prevailing mood in the Black and Yellow camp seems to be to play the match tomorrow for Marc."The team left its hotel about 7 p.m. local time (1 p.m. ET) for the UEFA Champions League quarterfinals match, a Dortmund police spokesman said.Between 40 and 50 people, including the trainer and medical support staff, were on board the bus. A preliminary investigation indicates the explosives "could have been hidden in a hedge near a parking lot," according to an update posted on the Dortmund police website.